From the viewpoint of global environment conservation, improvement in the fuel consumptions of automobiles has recently been required for satisfying the CO2 emission regulations. In addition, in order to secure safe of passengers at the time of crash, improvement in the safety of motor vehicle bodies has been also required mainly in consideration of the crashworthiness of vehicle bodies. In this way, weight lightening and strengthening of vehicle bodies have been positively advanced.
In order to simultaneously achieve weight lightening and strengthening of vehicle bodies, it is said to be effective that a part material is strengthened and the thickness of a part of sheet is decreased within a range which causes no problem of rigidity, and the weight is decreased by decreasing the thickness of a sheet. Therefore, high-tensile strength steel sheets have been recently positively used for automobile parts.
The weight lightening effect increases as the strength of the steel sheet used increases, and thus the car industry has the tendency to use steel sheets having a tensile strength (TS) of 440 MPa or more, for example, as panel materials for inner parts and outer parts.
On the other hand, many automobile parts made of steel sheets are formed by press forming, and thus steel sheets for automobiles are required to have excellent press formability. However, high-strength steel sheets are greatly inferior in formability, particularly deep drawability, to general mild steel sheets. Therefore, steel sheets having high deep drawability and a TS of 440 MPa or more, more preferably a TS of 500 MPa or more, and further preferably a TS of 590 MPa or more have been increasingly required for advancing weight lightening of vehicles. Also, high-strength steel sheets having a high Lankford value (referred to as a “r value” hereinafter), which is an evaluation index for deep drawability, for example, average r value ≧1.2, have been required.
As means for increasing strength while maintaining a high r value, Ti and Nb are added in amounts sufficient to fix carbon and nitrogen dissolved in ultra low carbon steel to form IF (Interstitial atom free) steel to be used as a base, and solid-solution strengthening elements such as Si, Mn, P, and the like are added to the base. This method is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 56-139654.
Japanese Unexamined Patent Application Publication No. 56-139654 discloses a technique for a high-strength cold rolled steel sheet having excellent formability, anti-aging properties, a tensile strength at the level of 35 to 45 kgf/mm2 (level of 340 to 440 MPa), and the composition: C: 0.002 to 0.015%, Nb: C %×3 to C %×8+0.020%, Si: 1.2% or less, Mn: 0.04 to 0.8%, and P: 0.03 to 0.10%. Specifically, this document discloses that a anti-aging high-strength cold-rolled steel sheet having a TS of 46 kgf/mm2 (450 MPa) and an average r value of 1.7 can be produced by hot rolling, cold rolling, and recrystallization annealing ultra low carbon steel used as a raw material and containing 0.008% of C, 0.54% of Si, 0.5% of Mn, 0.067% of P, and 0.043% of Nb.
However, it has been known that when a high-strength steel sheet having a tensile strength of 440 MPa or more or a higher tensile strength of 500 MPa or more or 590 MPa or more is produced by the technique of adding solid-solution strengthening elements to ultra low carbon steel used as a raw material, the amounts of the alloy elements added are increased to cause the problem of surface appearance, the problem of degrading plating performance, the problem of secondary cold-work embrittlement, and the like. Also, the addition of large amounts of solid-solution strengthening elements decreases the r value, thereby causing the problem that the r value level is decreased as strength is increased. Furthermore, in order to decrease a carbon content to the ultra low carbon region, such a C content of less than 0.010% as disclosed in the Japanese Unexamined Patent Application Publication No. 56-139654, vacuum degassing must be performed in a steel making process. This means that a large amount of CO2 is generated in a production process. Therefore, from the viewpoint of global environment conservation, it is difficult to say that this technique is a preferable technique.
Besides the above-described solid-solution strengthening method, a microstructure strengthening method can be used as a method for increasing the strength of a steel sheet. For example, a dual phase steel sheet (DP steel sheet) having a soft ferrite phase and a hard martensite phase is produced by this method. A DP steel sheet generally has characteristics, such as substantially excellent ductility, an excellent strength-ductility balance (TS×E1), and a low yield ratio (YS/TS). In other words, the DP steel sheet has characteristics, such as a low yield ratio for the tensile strength and excellent shape fixability in press forming. However, the steel sheet has a low r value and unsatisfactory deep drawability. This is said to be due to the fact that dissolved C, which is essential in forming a martensite phase, inhibits the formation of a {111} recrystallized texture effective in increasing the r value.
For example, Japanese Examined Patent Application Publication No. 55-10650 or Japanese Unexamined Patent Application Publication No. 55-100934 discloses a technique as an attempt to improve the r value of such a dual-phase steel sheet
Japanese Examined Patent Application Publication No. 55-10650 discloses a method including cold rolling, box annealing at a temperature of a discloses a method including cold rolling, box annealing at a temperature of a recrystallization temperature to an Ac3 transformation point, heating to 700 to 800° C. for forming a dual phase, and then quenching and tempering. However, this method includes quenching and tempering in continuous annealing, and thus has the problem of production cost. Also, box annealing is inferior in treatment time and efficiency to continuous annealing.
The technique of Japanese Unexamined Patent Application Publication No. 55-100934 for achieving a high r value includes cold rolling, box annealing at a temperature in a ferrite (α)-austenite (γ) intercritical region, and then continuous annealing. In this technique, Mn is concentrated from a α phase to a γ phase in soaking for box annealing. Then, the Mn-concentrated phase is preferentially converted to the γ phase during continuous annealing, and thereby a mixed microstructure can be obtained by cooling even at a gas jet cooling rate. However, this method requires long-term box annealing at a relatively high temperature for concentrating Mn, and also requires a large number of steps. Therefore, the method has not only low economics from the viewpoint of production cost but also many problems with the production process, such as the adhesion of coiled steel sheets, the occurrence of a temper color, a decrease in life of a furnace inner cover, and the like.
Japanese Examined Patent Application Publication No. 1-35900 discloses a process for producing a dual-phase high-strength cold-rolled steel sheet having excellent deep drawability and shape fixability, in which steel containing 0.003 to 0.03% of C, 0.2 to 1% of Si, 0.3 to 1.5% of Mn, and 0.02 to 0.2% of Ti ((effective Ti/(C+N)) atomic concentration ratio of 0.4 to 0.8) is hot-rolled, cold-rolled, and then continuously annealed by heating to a predetermined temperature and then rapidly cooling. Specifically, the document discloses that steel having a composition including, % by mass, 0.012% of C, 0.32% of Si, 0.53% of Mn, 0.03% of P, and 0.051% of Ti is cold-rolled, heated to 870° C. in a α-γ intercritical region, and then cooled at an average cooling rate of 100° C./s to produce a dual-phase cold rolled steel sheet having a r value of 1.61 and a TS of 482 MPa. However, a water quenching apparatus is required for achieving a cooling rate of as high as 100° C./s, and a problem with surface treatment properties of a water-quenched steel sheet is actualized, thereby causing problems of production equipment and material quality.
Japanese Unexamined Patent Application Publication No. 2002-226941 discloses a technique for improving the r value of a dual-phase steel sheet by optimizing the V content in relation to the C content. In this technique, C contained in steel is precipitated as a V-based carbide to minimize the amount of dissolved C before recrystallization annealing, thereby achieving a high r value. Then, the steel is heated in the α-γ intercritical region to dissolve the V-based carbide and concentrate C in the γ phase, and then cooled to produce a martensite phase. The addition of V increases the cost because V is expensive, and VC precipitated in the hot-rolled sheet increases deformation resistance in cold rolling. Therefore, for example, in cold rolling with a reduction ratio of 70% as disclosed in an example, a load on a roll is increased to cause the problems with production, such as an increase in the danger of occurrence of a trouble and the possibility of decreasing productivity.
Furthermore, Japanese Unexamined Patent Application Publication No. 2003-64444 discloses a technique as a technique for a high-strength steel sheet having excellent deep drawability and a process for producing the same. This technique is aimed at producing a high-strength steel sheet having a predetermined C content, an average r value of 1.3 or more, and a microstructure containing at least one of bainite, martensite, and austenite in a total of 3% or more. The process for producing the steel sheet includes cold rolling with a reduction rate of 30 to 95%, annealing for forming Al and N clusters and precipitates to develop a texture and increase the r value, and then heat treatment for causing the texture to contain at least one of bainite, martensite, and austenite in a total of 3% or more. This method requires annealing for achieving a high r value after cold rolling and then heat treatment for obtaining the texture, and the annealing step basically includes box annealing and requires a long holding time of 1 hour or more, thereby causing the problem of low productivity of the process (processing time). Furthermore, the resultant texture has a relatively high second phase fraction, and thus it is difficult to stably secure an excellent strength-ductility balance.
The conventional method for increasing strength by solid-solution strengthening, which has been conventionally investigated, requires the addition of large amounts or excessive amounts of alloy elements for increasing the strength of a (mild) steel sheet having excellent deep drawability, and thus the method has problems with the cost and process and problems with improvement in the r value.
The method utilizing microstructure strengthening requires two times of annealing (heating) and high-speed cooling equipment, and thus has problems with the production process. Although the method utilizing VC is also disclosed, the addition of expensive V increases the cost, and the precipitation of VC increases deformation resistance in rolling, thereby causing difficulty of stable production.
Thus, it could be helpful to resolve the problems of the conventional methods and provide a high-strength steel sheet having a TS of 440 MPa or more, an average r value ≧1.2 and excellent deep drawability, and a production process therefor. It could also be helpful to provide a high-strength steel sheet having a high average r value of 1.2 or more and excellent deep drawability while maintaining high strength, such as TS ≧500 MPa or TS ≧590 MPa, and a production process therefor.